Rate that coffee is being added to the urn after 4 hours is 3(4)^3 + 20 = 212 cups/hr
Rate that coffee is being dispensed from the urn after 4 hours = 5(4)^2 + 10 = 90 cups/hr
So
(e - l)(4) = 212 - 90 = 122 [ this means that after 4 hours, coffee is being added to the urn at 122 cups / hr ]